This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Life threatening diarrhea affects approximately 4 billion people per year worldwide with number of deaths estimated to be around 2 million. Enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC, respectively) that attach to apical surface of the intestinal epithelium and cause its effacement, are common etiological agents for infectious diarrhea worldwide, especially in children less than 2 years of age. Citrobacter rodentium infection in mice is used as an in vivo model system for EPEC and EHEC infections in humans. Considerable variability in susceptibilities to infection and severity of the disease exists among mice based on their strain and age. We hypothesize that the colonic epithelium plays a central role in determining the clinical presentation of C. rodentium infections in mice. In this study we will assess functional and structural alterations in epithelial barrier following infection with C. rodentium under in vitro and in vivo conditions. Immortalized colonic epithelial monolayer and the colons from the highly susceptible (FVB and C3H) and resistant (Swiss Webster) mice strains will be infected with C. rodentium and changes in transepithelial resistance, paracellular permeability, tight junctional integrity, cellular architecture, and profiles of secreted cytokines will be evaluated. This study will help us understand how the colonic mucosal epithelium influences the host-pathogen interactions to define the course and progression of illnesses caused by diarrheagenic gastrointestinal bacterial pathogens including EPEC and EHEC in humans.